This invention relates to ring laser gyroscopes in general and more particularly to new and novel circuitry for providing phase lock between the dither drive motion of the master and slave units as well as phase lock between the dither drive motion and the motion imparted by the path length controller drivers.
Because of the complex requirements of today's military and space flight equipment, greater and greater demands are being placed on such equipment as guidance systems. Since gyroscopes represent an essential part of most such systems, these stringent demands are also required of the gyroscope itself. Therefore, over the years many types of gyroscopes have been developed to meet there increasing demands. One sophisticated modern type gyroscope is referred to as "a ring laser gyroscope". As is inherent in its name, the ring laser gyroscope uses laser beams with travel in a closed path. Regardless of whether the closed path is triangular, square, octagonal, etc., the closed path is commonly referred to as a ring. Such a ring laser gyroscope is used to detect rotation about the axis of the path around which the laser beam travels. Typical ring laser gyroscopes are disclosed in U.S. Pat. Nos. 3,373,650 and 3,467,472.
These gyroscopes include a triangular block into which is bored a triangular-shaped cavity defined by mirrors at the three corners. It will be appreciated that the triangular cavity is preferred since it requires a minimum number of mirrors. The laser cavity itself is filled by a gas which comprises, for example, helium and neon. Through proper choice of the ratios of the neon isotopes in the gas mixture, two monochromatic laser beams are created. The two laser beams travel in opposite directions around the triangular cavity in the same closed optical path.
With no angular motion about the input axis of the ring laser gyroscope, the lengths of the two laser beams are equal, and the two optical frequencies are the same. Angular motion causes an apparent increase in the cavity length for the beam traveling in the direction of such angular movement and a corresponding decrease for the beam traveling in the opposite direction. Because the closed optical path is a resonant cavity providing sustained oscillation, the wave length of each beam must also be increased or decreased and therefore each beam frequency changes accordingly. Thus, a frequency differential occurs between the two beams which is proportional to the angular rate.
According to certain prior practices, the two beams are extracted from the laser at its output mirror and they are heterodyned in a beam combiner to produce an interference pattern. The interference pattern is detected by a photodetector which senses the difference frequency of the two beams, this frequency being a measure of the angular rate.
It is not uncommon, that at low angular rates, the frequency differential between the two beams is so small that the beams tend to resonate together, or "lock in" so that the two beams oscillate at only one frequency. If this occurs it is impossible to read the change in the angular rate because the frequency differential proportional to the angular rate does not exist. The angular rate below which "lock in" occurs is termed the lock-in rate. One means to reduce lock-in effects is to apply a sinusoidal input rate which is many times the lock-in rate, thus preventing lock-in except for short periods of time when the instantaneous sinusoidal input rate is less than the lock-in rate. This sinusoidal rate, commonly referred to as "dither" is typically imparted to the ring laser gyroscope by means of a PZT (Piezoelectric-Transducer) attached to the flexible suspension supporting the ring laser gyroscope block. These PZT's are typically driven by signals having a random component as well as a sinusoidal component.
The flexible suspension supporting the ring laser gyroscope block forms a resonant system having a resonant frequency of typically 250 Hz and a Q value which can vary from less than 100 to several hundred. Those skilled in the art recognize the desirability of providing dither drive at the resonant frequency of the dither suspension system in order to achieve the required dither motion with the least amount of drive energy. This can be done by connecting the dither suspension system in an oscillator loop wherein it becomes the element which determines the frequency of oscillation of the dither system. One or more PZT's connected to the system provide a signal at the frequency of oscillation to an amplifier which in turn drives other PZT's to sustain the oscillation.
In a system comprising three or more ring laser gyroscopes each could be so configured to independently oscillate at the resonant frequency of its dither suspension system. However, due to small differences in the tolerances of the parts which make up the dither suspension system, the resonant frequencies and their variations with temperature result in frequencies of oscillation which differ by small amounts among the ring laser gyroscopes. This produces beat frequencies and low frequency vibrations which are coupled among the ring laser gyroscopes adversely affecting performance. It is desirable, therefore, to synchronize the dither motions of all the ring laser gyroscopes.
According to one technique in the prior art, one of the three ring laser gyroscopes which typically make up a ring laser gyroscope system is selected as a master unit and provides an a-c frequency reference signal to the other two units which act as slave units, thus determining the frequency of oscillation of all three. However, a mechanical coupling exists between the dither motion of each of the ring laser gyroscopes due to the finite weight and suspension stiffness of the support block and unfortunately small changes in that stiffness and damping is believed to be the cause of undesirable phase shifts among the dither motions of the ring laser gyroscopes. Since the prior art dither drive circuitry simply provides an a-c frequency reference signal, such prior art systems do nothing to alleviate this serious problem.
Therefore, to overcome the shortcomings of presently available methods and apparatus, it is an object of this invention to provide methods and apparatus for phase locking the dither motion of the separate ring laser gyroscopes comprising a system.
It is still another object of this invention to provide inexpensive and simple circuitry for generating dither motion.
It is a further object of this invention to provide circuitry for locking in phase the path length control motion of a ring laser gyroscope with the dither motion.
To accomplish the above mentioned objects as well as other objects which will become evident from the following drawings and detailed description, the present invention provides circuitry for use with a ring laser gyroscope system having a multiplicity of ring laser gyroscope units wherein one of the ring laser gyroscopes is selected to operate as the master unit and the remaining units are selected to operate as slave units to provide phase lock between the dither drive motion of the master unit and the slave units.
The circuitry includes a multiplicity of circuits, one each for use with one each ring laser gyroscope, each of the circuits comprising a means for sensing the dither motion of its respective ring laser gyroscope and for providing an electrical dither signal representative of such dither motion. A dither signal is provided to a phase detector which in turn provides a phase difference output signal which varies in response to said dither signal and a control signal. The phase difference signal is provided to a variable oscillator which also receives a random input noise signal in turn provides a drive signal having a frequency which varies in response to the random input signal and the phase difference signal. The drive signal is provided to a means for imparting the dither motion to an appropriate ring laser gyroscope such that the dither motion varies in response to the applied drive signal. The drive signal provided by the ring laser gyroscope selected to be the master signal is also connected to each of the phase detectors to act as the control signal for the slave units. Thus, it can be seen that the dither motion of each of the ring laser gyroscopes will be in phase lock with the drive signal provided by the circuitry corresponding to the master unit.